Anurag Agarwal

Calculation of sound propagation in nonuniform flows: Suppression of instability waves

Acoustic waves propagating through nonuniform flows are subject to convection and refraction. Most noise prediction schemes use a linear wave operator to capture these effects. However, the wave operator can also support instability waves that, for a jet, are the well-known Kelvin-Helmholtz instabilities. These are convective instabilities that can completely overwhelm the acoustic solution downstream of the source location. A general technique to filter out the instability waves is presented. A mathematical analysis is presented that demonstrates that the instabilities are suppressed if a time-harmonic response is assumed, and the governing equations are solved by a direct solver in the frequency domain. Also, a buffer-zone treatment for a nonreflecting boundary condition implementation in the frequency domain is developed. The outgoing waves are damped in the buffer zone simply by adding imaginary values of appropriate sign to the required real frequency of the response. An analytical solution to a one-dimensional model problem, as well as numerical and analytical solutions to a two-dimensional jet instability problem, are provided. They demonstrate the effectiveness, robustness, and simplicity of the present technique.

Flow decomposition and aerodynamic sound generation

An approximate decomposition of fluid-flow variables satisfying unbounded compressible Navier–Stokes equations into acoustically radiating and non-radiating components leads to well-defined source terms that can be identified as the physical sources of aerodynamic noise. We show that, by filtering the flow field by means of a linear convolution filter, it is possible to decompose the flow into non-radiating and radiating components. This is demonstrated on two different flows: one satisfying the linearised Euler equations and the other the Navier–Stokes equations. In the latter case, the corresponding sound sources are computed. They are found to be more physical than those computed through classical acoustic analogies in which the flow field is decomposed into a steady mean and fluctuating component.

Low-Frequency Acoustic Shielding by the Silent Aircraft Airframe

The Silent Aircraft airframe has a flying-wing design with a large wing planform and a propulsion system embedded in the rear of the airframe with intake on the upper surface of the wing. In the present paper, boundaryelement calculations are presented to evaluate acoustic shielding at low frequencies. Besides the three-dimensional geometry of the Silent Aircraft airframe, a few two-dimensional problems are considered that provide some physical insight into the shielding calculations. Mean-flow refraction effects due to forward-flight motion are accounted for by a simple time transformation that decouples the mean-flow and acoustic-field calculations. It is shown that a significant amount of shielding can be obtained in the shadow region where there is no direct line of sight between the source and observer.

Mice produce ultrasonic vocalizations by intra-laryngeal planar impinging jets

Rodent ultrasonic vocalizations (USVs) are a vital tool for linking gene mutations to behavior in mouse models of communication disorders, such as autism [1]. However, we currently lack an understanding of how physiological and physical mechanisms combine to generate acoustic features of the vocalizations, and thus cannot meaningfully relate those features to experimental treatments. Here we test and provide evidence against the two leading hypotheses explaining USV production: superficial vocal fold vibrations [2], and a hole-tone whistle [3]. Instead, we propose and provide theoretical and experimental evidence for an alternative and novel vocal production mechanism: a glottal jet impinging onto the laryngeal inner planar wall. Our data provide a framework for future research on the neuromuscular control of mouse vocal production and for interpreting mouse vocal behavior phenotypes.

Trailing edge noise theory for rotating blades in uniform flow

This paper presents a new formulation for trailing edge noise radiation from rotating blades based on an analytical solution of the convective wave equation. It accounts for distributed loading and the effect of mean flow and spanwise wavenumber. A commonly used theory due to Schlinker and Amiet predicts trailing edge noise radiation from rotating blades. However, different versions of the theory exist; it is not known which version is the correct one, and what the range of validity of the theory is. This paper addresses both questions by deriving Schlinker and Amiets theory in a simple way and by comparing it with the new formulation, using model blade elements representative of a wind turbine, a cooling fan and an aircraft propeller. The correct form of Schlinker and Amiets theory is identified. It is valid at high enough frequency, i.e. for a Helmholtz number relative to chord greater than one and a rotational frequency much smaller than the angular frequency of the noise sources.

The Calculation of Acoustic Shielding of Engine Noise by the Silent Aircraft Airframe

The Silent Aircraft airframe has a flying wing design with a large wing planform and a propulsion system embedded in the rear of the airframe with intake on the upper surface of the wing. In the present paper, boundary element calculations are presented to evaluate acoustic shielding at low frequencies. Besides the three-dimensional geometry of the Silent Aircraft airframe, a few two-dimensional problems are considered that provide some physical insight into the shielding calculations. Mean flow refraction effects due to forward flight motion are accounted for by a simple time transformation that decouples the mean-flow and acoustic-field calculations. It is shown that significant amount of shielding can be obtained in the shadow region where there is no direct line of sight between the source and observer. The boundary element solutions are restricted to low frequencies. We have used a simple physically-based model to extend the solution to higher frequencies. Based on this model, using a monopole acoustic source, we predict at least an 18 dBA reduction in the overall sound pressure level of forward-propagating fan noise because of shielding.

Prediction Method for Broadband Noise from Unsteady Flow in a Slat Cove

Noise from high-lift devices such as slats and flaps can contribute significantly to the overall aircraft sound pressure levels, particularly during approach. The acoustic spectrum of the noise radiated from slats exhibits two distinct features. There is a high-frequency tonal noise component and a high-energy broadband component ranging from low- to midfrequencies. The objective is to predict the broadband slat noise. The broadband noise is predicted using a two-step process. First, the noise sources are modeled based on the local turbulence information. Then, the sound from these sources is propagated by assuming that the flow past the wing is uniform. A boundary element method is used to find Green’s function for wave propagation in a moving medium in the presence of the wing. The noise in the far field is then predicted by forming a convolution of Green’s function with the modeled sources. The attractive feature of this prediction scheme is the quick computational time, which makes it suitable for new design and control strategies.

A ray tracing approach to calculate acoustic shielding by the Silent Aircraft airframe

The Silent Aircraft is in the form of a flying wing with a large wing planform and a propulsion system that is embedded in the rear of the airframe with intakes on the upper surface of the wing. Th ...

BROADBAND NOISE FROM THE UNSTEADY FLOW IN A SLAT COVE

Noise from high-lift devices such as slats and aps can contribute signicantly to the overall aircraft sound pressure levels, particularly during approach. The acoustic spectrum of the noise radiated from slats exhibits two distinct features. There is a high-frequency tonal noise component, and a high-energy broadband component ranging from low to mid-frequencies. The objective of the present paper is to predict the broadband slat noise. The broadband noise is predicted using a two-step process. First the noise sources are modeled based on the local turbulence information. Then, the sound from these sources is propagated by assuming that the ow past the wing is uniform. A Boundary Element Method is used to nd the Green’s function for wave propagation in a moving medium in the presence of the wing. The noise in the far eld is then predicted by forming a convolution of the Green’s function with the modeled sources. The attractive feature of this prediction scheme is the relatively quick computational time, which makes it suitable for new design and control strategies.

Nonlinear and linear noise source mechanisms in subsonic jets

Noise source mechanisms are studied for a numerical dataset of a low Reynolds number laminar jet with a Mach 0.9 jet exit velocity (Suponitsky et al., J. Fluid Mech., Vol. 658, 2010) and two experimentally obtained datasets of high Reynolds number, Mach 0.4 and 0.6 turbulent jets (Cavalieri et al., AIAA Vol. 2011-2743, 2012). The objective of the study is to discern the source mechanism, linear or non-linear, by which acoustic radiation is obtained from wave-packets in the context of laminar and turbulent jets. For the laminar jet, it is shown numerically using a Linearized Euler Equation (LEE) solver that the sources of sound stem from a non-linear coupling of hydrodynamic waves. The nonlinear nature of the source mechanism explains why Linear Parabolized Stability Equation (LPSE) formulations are unable to reproduce the relevant near field dynamics at low Reynolds numbers. For the turbulent jets however, experimental evidence indicates that linear wavepackets are likely to be the source mechanism for acoustic radiation. To verify this, a fluctuating boundary condition is incorporated into the LEE solver such that a single frequency hydrodynamic wave is set up. This is used to investigate how the results from linear wavepackets compare with those found from LPSE and experiments. It is found that the power spectral density of the axial velocity fluctuations obtained by LEE shows a close match with those obtained from the LPSE and experiments, and it is also observed that downstream of the potential core, LEE results match more closely with experiments in this regard than do LPSE results. However, although the linear wavepackets formed using a fluctuating boundary condition do radiate sound, a comparison of the far-field directivity results show that the amplitude of the sound produced is significantly lower than those observed in experiments. Based on these results, LEE with a fluctuating boundary condition proves to be more useful in reproducing the near flow field of a turbulent jet but does not appear to be accurate in directly predicting the radiated far-field sound.